Let $a,$ $b,$ $c,$ $d$ be real numbers such that
\[a^2 + b^2 + c^2 + d^2 = 4.\]Find the maximum value of $a^3 + b^3 + c^3 + d^3.$
Explanation: From the equation $a^2 + b^2 + c^2 + d^2 = 4,$ $a^2 \le 4,$ so $a \le 2,$ or $2 - a \ge 0.$  Then
\[(2 - a) a^2 \ge 0,\]so $a^3 \le 2a^2.$  Similarly, $b^3 \le 2b^2,$ $c^3 \le 2c^2,$ and $d^3 \le 2d^2.$  Adding all these inequalities, we get
\[a^3 + b^3 + c^3 + d^3 \le 2(a^2 + b^2 + c^2 + d^2) = 8.\]Equality occurs when $a = 2$ and $b = c = d = 0,$ so the maximum value is $\boxed{8}.$